1. Field of the Invention
The present invention relates to coffee brewing apparatuses and particularly to a piston-cylinder-assembly to be used in a coffee brewing apparatus. Even more particularly, the invention relates to a piston-cylinder-assembly comprising a cylindric cavity serving as a brewing chamber, a closing member mounted on the brewing chamber at one end thereof and a piston adapted to be displaced into and retracted from the interior of the brewing chamber at the other end of the brewing chamber, said piston including a brewing water connector member and internal channels leading from the brewing water connector member to the front face of the piston. Forthermore, the invention relates to a method of operating the above mentioned piston-cylinder-assembly.
2. Prior Art
Coffee brewing apparatuses incorporating a similar piston-cylinder-assembly are known in the art, e.g. from the Patent Publication PCT-WO No. 82/01120 and the German Laid Open Publication No. 20 53 466.
The apparatus disclosed in the publication PCT-WO No. 82/01120 comprises a cylinder and two pistons which are displaceable independently from each other and in opposite directions. The coffee powder to be scalded is contained between the two pistons. In operation, the coffee powder is filled into the space between the two pistons and steeped with hot water when it is still loose to allow it to swell. Thereafter, one of the two pistons is further displaced to compact the coffee powder and hot water is forced through the compacted coffee powder to prepare the beverage. In this way, two objects should be met. On one hand, an aromatic strong coffee should be obtained with a small quantity of coffee powder; on the other hand, the quantity of coffee powder should be dosed thanks to the two pistons which are displaceable independently from each other in order to select the intensity of the taste of the coffee.
However, this publication does not disclose how to adjust or dose the amount of coffee powder by means of the two independently movable pistons. Moreover, the disclosure of this publication is limited to the teaching that two independently movable pistons are provided to close the brewing chamber, that the still loose coffee powder is scalded in a first phase and that the coffee powder is subsequently compacted to prepare the desired beverage.
It is possible that an improved exploitation of the coffee powder may be achieved, but the apparatus according to the aforementioned publication is disadvantageous insofar as its design is quite complicated due to the provision of two independently displaceable pistons and that the preparation cycle takes a comparatively long time so that one has to wait unduly long until a cup of coffee is prepared.
Also the structure of the coffee brewing apparatus according to the German Laid Open Publication No. 20 63 466 comprises two pistons. While the upper piston may be retracted out of the brewing chamber in order to enable the brewing chamber to be filled with coffee powder to be scalded and serves to close the brewing chamber as well as to compact the coffee powder contained therein, the lower piston serves as closing element of the brewing chamber and its position may be changed by a number of preset stops, obviously mainly to vary the internal volume of the brewing chamber, but also to alter the compacting pressure exerted on the coffee powder by the first displaceable piston.
The constructional expenditure of this structure is quite high as well and the desired object is not fully met since the compacting pressure and the internal volume of the brewing chamber can be altered but very roughly.